1. Field of the Invention
The present invention relates to a shiftable device which acts as a brake or coupling, such as a frictional brake or a frictional clutch. The device includes a first component, such as the stator of a brake or the first half of a clutch and a second component, such as the rotor of a brake or the second half of a clutch. A pressure ring is arranged between the first and second components, wherein the pressure ring is connected nonrotatably or stationary to the first component, while being movable by means of an actuating and restoring means, such as springs, an electric magnet and/or pressure means, between two work positions which are axially offset relative to each other. The first work position constitutes a first position of operation (active position), such as the engaged position of a brake, and a second position of operation (non-active position), such as a released position of the brake. At least one friction element which is compressed in the active position of the pressure ring is arranged between the pressure ring and the second component.
2. Description of the Related Art
Devices of the above-described type are used in servo-motors as frictional brakes which are required to precisely control drives in accordance with a certain movement pattern. The brake not only has to be effectively actuated at a certain time; rather, the brake must hold in its active position the rotor relative to the stator of the brake without play. When the brake is engaged, no further rotation may occur after standstill has been reached.
In known devices of this type, the requirements mentioned above can only be achieved through very expensive means. In a known device, constructed as an electromagnetically releasable spring pressure brake, the first component is a stator with integrated coil fixed to the machine frame, wherein an armature disk acting as pressure ring is attached through a diaphragm in an axially movable manner. The armature disk is under the influence of compression springs which have the tendency to press the armature disk against an axial friction element which is mounted on a brake disk arranged axially in front of the armature disk. The brake disk constitutes the second component which acts as a rotor and is connected to a shaft to be decelerated. By switching on the coil current, the armature disk is released electromagnetically from the friction element of the brake disk against the spring force acting on the armature disk, so that the released position of the brake is reached. The electromagnetic control only permits small axial movements of the armature disk between the active position and the released position thereof. Consequently, seen in the released position, the minimum air gap between the brake disk supporting the friction element, on the one hand, and the armature disk, on the other hand, must be adjusted very precisely. This requires a very complicated manufacture and time-consuming assembly of the structural components within very narrow tolerances. When wear occurs after longer periods of operation, complicated readjustments are required. The axial frictional engagement between the armature disk and the brake disk is determined exclusively by the spring force which, in the case of high braking torques, requires appropriately substantial spring elements. In the active position of operation of the brake, the braking torque is applied to the armature disk and must be transmitted axially to the stator of the brake through the diaphragm or the like supporting the armature disk.